The present invention pertains to a process for the production of a methane-containing gas having a methane content between 40 and 99% by volume from gases which contain, besides hydrogen and carbon dioxide, more than 20% by volume of carbon monoxide by catalytic conversion and methanation followed by washing to remove carbon monoxide therefrom.
Heretofore fuel gases rich in methane have been produced from gases containing hydrogen, carbon monoxide and carbon dioxide by use of nickel-containing catalysts in accordance with the following equilibrium equations: EQU CO.sub.2 + 4H.sub.2 .revreaction.CH.sub.4 + 2H.sub.2 O I. EQU co + 3h.sub.2 .revreaction.ch.sub.4 + h.sub.2 o ii.
both of the foregoing so-called methanation reactions (proceeding from left to right of the equations) are strongly exothermic. These reactions have long been used in industry to remove in this manner the carbon monoxide which is highly toxic for some catalysts from synthesis gas which is used for the synthesis of ammonia. Another important use of these reactions is in processes for the production of fuel gases which, because of their high contents of methane, can be used to replace natural gas.
Gases containing more than 20% by volume of carbon monoxide that can be treated in accordance with the processes of the present invention are produced in various known processes. Such gases are preferably produced by partially oxidizing and thereby gasifying solid or liquid fuels such as coal, lignite, peat, or heavy petroleum oils. They can also be produced by the catalytic cracking of hydrocarbons. It is also possible to use gases from other sources, such as, for example, so-called producer and water gases having suitable compositions.
Such gases containing carbon monoxide, carbon dioxide, and hydrogen, do not react solely in accordance with the foregoing equations I and II during methanation but also undergo a further conversion in a so-called water-gas reaction in accordance with the following equilibrium equation: EQU CO + H.sub.2 O.revreaction.CO.sub.2 + H.sub.2 III.
this reaction (proceeding from left to right), which is often referred to as the conversion reaction, and is so referred to herein simply as such, is likewise exothermic. However, the velocity of the reaction that is produced when nickel catalysts are used is smaller than that produced in the methanation reactions in accordance with equations I and II.
From the foregoing it can be assumed that it must be possible also to treat gases which contain little hydrogen and much carbon monoxide with nickel catalysts for simultaneous conversion and methanation of a portion of the carbon monoxide contained therein. The coupling of the methanation reaction in accordance with equations I and II with the conversion reaction in accordance with equation III is quite desirable because the water that is required for the conversion reaction is generated in the methanation reactions.
Experience acquired by actual practice has shown, however, that it was heretofore not possible satisfactorily to perform together both the methanation and the conversion reactions. It was further established that, even after a relatively short period, a deposit of carbon or soot would form on the nickel-containing methanation catalyst, thereby greatly impairing or suppressing its activity.